Microstrip transmission lines are often used to transmit signals between electrical circuits and devices. In high-speed, high-density digital applications, the transmitted signal is made up of a series of short pulses. When the pulses in the signal are very short in time, their shape distorts as they travel down the line. This distortion is a result of three different mechanisms, namely, (1) dispersion, that is, each frequency component travels at a different speed; (2) attenuation, that is, part of the signal is dissipated as heat; and (3) coupling (crosstalk), that is, energy from the signal in the main line is transferred to adjacent line(s). All three of these mechanisms distort the shape of the pulse and reduce its amplitude. However, the mechanism of coupling (crosstalk) also creates one or more unwanted signals on adjacent line(s), which can cause errors in the operation of the circuit.
Previously, the center conductor in a transmission line required relatively large spaces between adjacent center conductors in order to avoid the problems of coupling, pulse distortion, and crosstalk. This solution, however, makes the dimensions of the circuits much larger overall, increases the amount of energy lost, and reduces the net speed of the circuit operation. N. B. Rabbat (See: IBM Technical Disclosure Bulletin vol. 8, No. 5, pp. 1430-1435, Oct. 1975) describes a technique for reducing crosstalk in microstrip transmission lines using extra ground holes. Crosstalk was reduced by about 60% experimentally but did not eliminate it completely. In addition, additional fabrication effort is required to create the extra ground holes.
U.S. Pat. No. 4,441,088 describes a stripline cable where there is a plurality of signal carrying conductors embedded in a dielectric layers above and below the signal conductors are chosen to provide the transmission lines with a certain impedance value and so that the coupling between the lines is minimized. Since the transmission lines are embedded in a dielectric layer, it is very difficult to mount standard digital chip packages, making this type of interconnection unsuitable for many high-speed applications. In addition, embedding the signal carrying conductors in the dielectric requires additional fabrication effort.
Thus, non of the previous solutions for reducing crosstalk and coupling is totally satisfactory and a need still exists to enhance the performance of multilayer-multiconductor microstrip circuits. It is toward the fulfullment of this need that the present invention is directed.